1. Field of the Invention
This invention relates to a method for increasing the high temperature fretting wear resistance of many nickel, cobalt, and titanium alloys.
2. Description of the Prior Art
Fretting wear occurs on the mating surfaces of two parts in contact which are designed to be static, but, through transmitted forces such as vibration, oscillate relative to each other with a high frequency, low amplitude motion. Since each surface contains many microscopic asperities in contact, the fretting motion tends to cause local adhesion at these junctions, which may fracture, producing material transfer, wear debris, or both. If the metals in contact are similar, or their constituent elements have good mutual solubilities, the majority of the fretting wear will be adhesive in nature. Elevated temperature will greatly accelerate the process, as may large loads and high frequencies. If the mating surfaces become extremely abraded and gnarled, stress concentrations will result in wear scarring, and a reduction in fatigue strength will result (known specifically as fretting fatigue), causing failure of the part(s). Specific examples of materials subject to fretting wear include turbofan airfoil bladeroots, friction dampers, bearings on shafts with loose fits, and drive-coupling components.
Prior art efforts to overcome the onset of fretting have included providing anti-fretting coatings on the surfaces of the materials in contact. These coatings behave as soft metallic films, keeping the substrate surfaces from coming in contact and dissipating vibrational energy by intracoating shear mechanisms. Such prior art anti-fretting coatings for nickel, cobalt, and titanium alloys have been based on Cu-Ni or Cu-Ni-In compositions. Their success is well documented. However, at temperatures above about 1000.degree. F., accelerated oxidation will rapidly deteriorate the coatings, allowing the substrate surfaces to come in contact and fret.
In addition, conventional application methods rely primarily on thermally spraying the constituents onto the desired part. This has further disadvantages due to the line-of-sight nature of thermal spraying, and the detrimental effects this type of operation has (e.g. warping) on thin-gauge materials.